Zooming in as far as you can go on your map will ensure you get your boat as close the way point mark as possible. Looking at it extended, you can put your boat on the mark and "assume" that you are on the mark. Zoom in and you will see that you are way off.

Also, it really depends on the unit and the type of grid coordinates they are utilizing in reference to their mapping software. A 6 digit grid is only accurate to within 100 meters. An 8 digit grid is good to within 10 meters. A 10 digit grid is good to within 1 meter. In most higher end GPS modules, you are talking somewhere in the middle of 8 to 10. In other words, somewhere in the neighborhood of 3 to 30 feet. Keep in mind that this is in reference to marking a point on TOP of the water. The unit will have to compensate when you mark a spot on the bottom as now you are adjusting for cone angle on your transducer. Some shoot a 9 degree cone, some a 19 degree cone, and some are even higher. 3 to 30 feet is a pretty good guestimate of being on the mark or off the mark.